Subthreshold depression - concept, operationalisation and epidemiological data. A scoping review

Purpose: In diagnostic systems (e.g., DSM-5, ICD-10), depression is defined categorically. However, the concept of subthreshold depression (SD) has gained increasing interest in recent years. The purpose of the present paper was to review, based on a scoping review, the relevant papers in this field published between October 2011 and September 2020. Materials and methods: Of the 1,160 papers identified, 64 records could be included in further analysis. The scoping review was conducted using both electronic and manual methods. Results: The main result of the analysis is that the operationalisation criteria used are highly heterogeneous, which also leads to very heterogenous epidemiological data. Conclusions: Clear conclusions are not possible scrutinising the reported results. Most definitions seem to be arbitrary, with considerable overlap (e.g., between SD and minor depression). The review also revealed that the impact of SD on quality of life and related parameters appear to be in the range of the respective impact of major depression (MD) and therapeutic approaches might be helpful for SD and also for the prevention of conversion from SD to MD. Keeping the presented difficulties in mind, a proposal for the definition of SD is made in the present paper in order to facilitate the discussion leading to more homogeneous criteria.